Effects of agricultural alkaline substances on reducing the rainwater acidification: Insight from chemical compositions and calcium isotopes in a karst forests area

Abstract

Two-year chemical and stable Ca isotopic compositions (δ44/40Ca) of rainwater were detected in a typical karst virgin forest site (Maolan National Nature Reserved Park, Southwest China, MNNRP). The results show that the pH values and the ionic concentrations of rainwater samples vary considerably, and about half of them are defined as acidic rain (pH < 5.0). NH4+ is the predominant cation in rainwater with a volume weighted mean (VWM) value of 110 μmol L−1 (1.3−377 μmol L−1), and the second is Ca2+ with VWM value of 11.7 μmol L−1 (0.9–67.7 μmol L−1). SO42- and NO3- are the principal anions with VWM values of 51.8 μmol L−1 (9.8−203 μmol L−1) and 24.7 μmol L−1 (2.7−151 μmol L−1), respectively. Source identification shows that the NO3- and SO42- of rainwater are controlled by anthropogenic sources, and the agricultural activities and natural processes are the main sources of NH4+, while the part of Ca2+, K+ and Mg2+ originate from crustal inputs. The higher concentration of alkaline ions is a significant contributor to reduce rainwater acidification. Based on the reported data of previous literature, the stable Ca isotopes (δ44/40Ca) are applied to decipher the different sources. The three sources include as the carbonate weathering (low δ44/40Ca ∼ 0.6‰, low NH4+/Ca2+ and NO3-/Ca2+ ratios), the silicate weathering which refers to the dissolvable soil dirt minerals from both local and the surrounding place (high δ44/40Ca ∼ 0.9‰, low NH4+/Ca2+ and NO3-/Ca2+ ratios), and the anthropogenic source (mainly from fertilizers, moderate δ44/40Ca ∼ 0.8‰, high NH4+/Ca2+ and NO3-/Ca2+ ratios). This study highlights the broader applicability of the chemical and Ca isotopic method in tracing the sources (particularly agricultural sources) of rainwater solute and atmospheric mixing/trapping processes, and can also provide additional insight regarding the elemental biogeochemical cycle in karst areas.